Thanks guys,
I guess the placement of the hammers is obvious now you mention it. The recipricol of that may be the true analogy then: the hammering on the outside in that pattern causes the metal to mould around the spiral on the mandrel within. Not so easy if you make barrels out of potatoes.

The transducer stuff is one of the closest things to a scientific look we've had on this thread. Could the issue be resolved using that?

- Paul